Recently, for the purpose of automotive lightening and safety improvement, it has been getting under way to highly strengthen automobile component parts and raw materials employed for the same. Steel plates, one of the representative examples, too, are such that the employment ratio of high-strength steel plates has about come to heighten. However, because of the fact that high-strength steel plates are of high strength and are hard in general, the degree of forming freedom is small in the press formability; moreover, there are such problems that the configurational freezability of pressed products is poor, the dimensional accuracy of formed products is defective and the longevity of pressing dies is short. Against these assignments, the improvements, which begin with the raw materials, have been getting under way as well. Recently, for the purpose of obtaining much-higher-strength component parts while providing them with good configurational accuracy, hot-working or hot-pressing technologies have become widespread, hot-working or hot-pressing technologies in which a steel plate is heated to 800° C. or more to soften it, is cooled rapidly simultaneously with press forming, and is quenched to make a high-strength component part; moreover, cold working-quenching technologies have come to be employed as an industrial technology, cold working-quenching technologies in which it is similarly quenched to make a high-strength component part after it is cold worked.
Meanwhile, since industrial machines, which are represented by automobiles, are such that the corrosion resistance in service environments is required sufficiently, components has been employed currently, component parts which are made by cold forming zinc-based plated steel plates, which are low cost and are good in terms of the corrosion resistance; however, in addition to this, many inventions have been known publicly, inventions in which surface-treated steel materials are heated to quench them.
In Patent Literature No. 1, a production method for a high-strength formed component part is disclosed, high-strength formed component part in which a zinc or zinc-alloy coating film is formed in a thickness of 5 μm-30 μm on a steel plate so that the protection against corrosion and decarburization, and the lubricational function are secured. In Patent Literature No. 2, a steel plate for hot pressing is disclosed, steel plate in which a barrier layer, which inhibits the volatilization of zinc upon heating, is formed on a zinc-plated layer before heating for quenching treatment. In Patent Literature No. 3, a hot-pressing method for a zinc-system-plated steel plate is disclosed. In Patent Literature No. 4, a hot-pressed formed product in which an iron-zinc solid-solution layer exists is disclosed.
However, in accordance with these methods, although all of them are better in terms of the corrosion than that of molded products, which are made by subjecting plating-free iron to quenching treatment, the corrosion resistance is still insufficient compared with that of molded products, which are made of plated steel plates being formed by ordinary cold working. It is because zinc volatilizes by means of heating. Against this problem, although aluminum-plated steel plates have been employed for applications in which the corrosion resistance, being equivalent to that of ordinary plated steel plates, is required, not only their costs are high but also the after-quenching corrosion resistance lowers more than that of cold-formed members none the less.
In accordance with aforementioned Patent Literature No. 2, before the quenching treatment, a barrier layer, which comprises an oxidized coating film, is formed on the zinc-plated layer of the steel material. In this case, when heating the steel material to a quenching temperature, or when heating it to the quenching temperature and holding it thereat, there is a fear that cracks might generate in the zinc-plated layer considerably because of the thermal expansion difference between the barrier layer, which has been formed originally on the steel material, and the zinc-plated layer. In this case, due to the cracks, the fear that the volatilization amount of zinc increases is highly likely, and it is not necessarily sufficient in order to obtain a plated layer whose corrosion resistance is good after the quenching treatment.
Against these problems, a technique has been desired strongly, technique which makes it possible to highly strengthen quenching and improve corrosion resistance in zinc-system-plated steel materials, which are more predominant in view of cost.
Patent Literature No. 1: Japanese Unexamined Patent Publication Gazette (KOKAI) No. 2001-353,548
Patent Literature No. 2: Japanese Unexamined Patent Publication Gazette (KOKAI) No. 2003-73,774
Patent Literature No. 3: Japanese Unexamined Patent Publication Gazette (KOKAI) No. 2003-126,920
Patent Literature No. 4: Japanese Unexamined Patent Publication Gazette (KOKAI) No. 2003-126,921
The present invention, in view of the aforementioned problems, is for providing a high-strength quenched formed body, which is good in corrosion resistance, in formed-body steel materials, to which the zinc-system-plated steel material, being predominant cost-wise, is performed and in which the corrosion resistance of after-quenching formed-body steel material is made equivalent to or more than that of cold-formed product, and a production process for the same.